In skin treatment, generally speaking, a skin treatment device contains an active treatment component that acts on the skin. This component, in operation, performs a treatment action on the skin that constitutes the actual treatment. The action may be mechanical, such as for instance in the case of shaving devices, depilation devices, mechanical epilation devices, cleaning devices, skin rejuvenation devices, etc. The action may be thermal, such as for instance in the case of optical depilation devices where a light beam is applied to the skin in order to affect part of the hair follicles. The action may be optical. In the case of optical depilation devices, the prior art comprises flash-type embodiments that generate one light flash or a limited number of flashes, and the user is supposed to hold the device steady during flashing and to displace the device in between flashes. The present invention, by contrast, relates to continuously operating devices that need to be moved over the skin surface continuously. It is noted in this respect that “continuously operating” includes the situation where a device is continuously generating light flashes, so that the device has to be displaced while performing the treatment.
In most, if not all, cases where the device has to be moved over the skin whilst in operation, optimum results require an optimum displacement speed. It will easily be understood that the treatment may be less effective if the displacement speed is too high, but the treatment may also be less effective if the displacement speed is too low, or worse, damage could be done to the user's skin if the device is displaced too slowly or, even worse, is held still.
It is true that the user manual may inform the user of the optimum speed or optimum speed range, but a problem for the users is how they should determine that they are actually moving the device at the right speed. In practice, this means that users have to go through a lengthy learning process before they have developed a “feeling” for the correct speed and are able to apply the correct speed consistently.
To assist the user in this learning process, solutions have already been proposed, according to which the device is provided with a speed sensor for detecting the relative speed between the device and the skin, and wherein the device is provided with an indicator giving the user feedback information. If the user is moving the device at a correct speed, information of a first type (or no information) is given to the user, but if the user is moving the device either too fast or too slow, information of a second type is given to the user, which will allow the user to adapt the speed and move slower or faster, respectively. For assisting the user in this way, the type of feedback signal is not essential.
For instance, the feedback signal may be a sound signal, or a tactile signal, such as vibrations, or an optical signal. US-2007/0129771 discloses a device where the indicator gives an optical signal.
However, it has been found to be very difficult for the user to handle the device at the correct speed. A feedback system may help by informing the user that the current speed is out of range, and although such a feedback system may be very simple, it appears to be very difficult for the user to move the device at the correct speed. Especially when the device is new to the user, the effect of the feedback signals may be less than expected a priori. The user will go too fast or too slow many times, and will hence receive many feedback signals pointing out these mistakes. With time and practice, the user's performance will improve, and the amount of “error signals” will decrease, but the user needs to be patient. However, it is quite possible that a user gets frustrated by receiving so many “error signals” and by not experiencing improvement quickly enough, which causes the user to give up and stop using the device altogether.